1. Field of the Invention
The present invention relates to a method of forming a weld joint between an austenitic stainless steel and a ferritic steel. The steels as joined are used to form part of a superheater or a reheater for a power boiler, which is subject to high temperature and high pressure.
2. Description of the Related Art
Two different metals, a low chromium-molybdenum alloy ferritic steel and an austenitic stainless steel, used in such a power boiler are welded together by a welding material such as D309 and inconel.
The low chromium-molybdenum alloy ferritic steel typically contains 0.15% or less carbon, 0.50% or less silicon, 0.30 to 0.60% manganese, 0.030% or less phosphorus, 0.030% or less sulphur, 1.90 to 2.60% chromium, and 0.87 to 1.13% molybdenum (see JISG3462STBA24). The austenitic stainless steel typically contains 0.04 to 0.10% carbon, 1.00% or less silicon, 2.00% or less manganese, 0.030% or less phosphorus, 0.030% sulphur, 9.00 to 13.00% nickel, 17.00 to 20.00% chromium, and 8.times.C% to 1.00 niobium (see JISG3463SUS347HTB).
The ferritic steel (coefficient of thermal expansion: about 13.times.10.sup.-6 /.degree.K.) and the austenitic stainless steel (coefficient of thermal expansion: about 17.times.10.sup.-6 /.degree.K.) have different coefficients of thermal expansion. A portion of the ferritic steel adjacent to the joint suffers from creep or creep fatigue fracture caused by thermal stresses since the boiler tends to employ high temperature, high pressure steam and is operated and stopped on a daily or weekly basis. As this occurs, the boiler is subject to malfunction. The JISG3462STBA24 ferritic steel has a creep-rupture strength of 2.2 kgf/mm.sup.2 when exposed for 100,000 hours at 600.degree. C. (see data provided by Metal Laboratory in the Bureau of Science and Technology, NRIM).